<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button class="begin">开启定时器</button>
    <button class="stop">停止定时器</button>
    <script>
        var timer=null;//全局变量 最好要赋值
        var begin=document.querySelector('.begin')
        var stop=document.querySelector('.stop')
        begin.addEventListener('click',function(){
            timer=setInterval(function(){
                console.log('123');
            },1000);
        })
        stop.addEventListener('click',function(){
            clearInterval(timer);
        })
    </script>
</body>
</html>